Is there/should there be a way to remove user accounts that haven’t been activated within a a certain time period?

I find that people sometimes register accounts with a wrong email or username and instead of changing those they simply reregister leaving the old account unactivated. Or their activation email doesn’t arrive fast enough or at all (especially a problem with hotmail) and they try another, leaving the old one behind.

It isn’t a huge problem, but those account come up with search and @name mentions.